Progress in Understanding Uranium(VI) Reduction Mechanisms in Natural Aquifers
Tuesday, September 11, 2012
John Bargar, Ph.D.
Stanford University - US Department of Energy
Environmental Geochemist and Senior Staff Scientist
at the SLAC National Accelerator Laboratory
Abstract: Microbially-driven metal reduction can profoundly alter uranium speciation and dynamics in aquifers. Under conditions where natural or amended organic carbon is present and dissolved oxygen is low or absent, bioreduction of U(VI) produces U(IV) and effects orders-of-magnitude decreases in uranium groundwater concentrations. Naturally (bio)reduced uranium occurs in contaminated aquifers at US Department of Energy legacy ore processing sites in the Western U.S., such as the Old Rifle, CO site, where it is has been implicated as a factor maintaining persistent uranium groundwater plumes. Stimulated bioreduction of groundwater uranium via organic carbon amendment is also of interest as a method to manage uranium redox state and mobility in impacted aquifers. While biological and abiotic reduction mechanisms have been studied in simplified laboratory systems, little is known about the mechanisms of U(VI) reduction in aquifers. Obtaining such information in aquifers is challenging, due to chemical and physical complexity of aquifer systems, low concentrations (< 1 ppm) of uranium, and the inaccessibility of the subsurface.
We have developed an in-situ approach to studying U(VI) reduction dynamics in the Old Rifle, Co, aquifer. This technique uses sediment columns submerged below the water table in existing wells to directly access biogeochemical conditions in aquifer sediments and thus to interrogate the chemical and physical forms of sediment-hosted U(IV) formed during acetate amendment field experiments. Harvested sediments were examined using x-ray and electron microscopy, x-ray absorption spectroscopy (XAS), and chemical extractions. U(IV) was found to occur as monomeric U(IV) complexes and as uraninite. Grain coating mineralogy and the distribution of U(IV) in the sediments differ strikingly following reduction under metal- and sulfate-reducing conditions. In contrast, U(IV) speciation is relatively constant under these differing biogeochemical regimes. This work establishes the importance of both monomeric U(IV) complexes and uraninite in subsurface sediments at the Old Rifle site. The body of this work provides a foundation from which the formation, dynamics, and stability of naturally reduced U(IV) in the Old Rifle aquifer can be investigated.